Optical encoders are electromechanical devices used typically for measuring position or monitoring motion and controlling the position with high precision. They are widely used in applications where extremely accurate linear or angular positioning information is required such as in computer-controlled manufacturing, factory automation, or land surveying equipment. Rotary optical encoders sense angular position by detecting laser light interrupted by markings making up coded tracks embedded on a disk. The disk rotates in sync with the movement being monitored. Rotary encoders are generally classified in either one of two categories i.e. as Absolute or Incremental encoders. Absolute encoders read a distinct coded track on the disk in order to determine a unique position and thus have the advantage of being able to determine the absolute position after a power failure without having to return to a “home” or index position. Incremental encoders can detect only relative position thus the position count can be “lost” in the event of a power failure or by severe jarring resulting in inadvertent disk movement. The advantage of being able to obtain the absolute position at any time makes absolute encoders desirable for many applications.
Advances in computer automated manufacturing technology require improvements in positioning precision to save space, cost and time. Naturally, the demand for high-resolution encoders capable of providing even finer incremental linear or rotary motion is greater still. Attempts at developing high-resolution encoders have been described in the prior art. Included in these are rotary encoders that are able to measure both the absolute and incremental angular position simultaneously from separate tracks embedded on a glass disk. The tracks correspond to an absolute track and an incremental track that are read cooperatively to determine an absolute position. Prior art absolute encoders that employ a single track containing both absolute and incremental position information are generally known to be less accurate that those using separate tracks.
An example of a prior art absolute encoder is given in U.S. Pat. No. 5,235,181 entitled: “ABSOLUTE POSITION DETECTOR FOR AN APPARATUS FOR MEASURING LINEAR ANGULAR VALUES.” It describes a position detector where the position is detected by reading separate absolute and incremental tracks. The absolute track is made up of pseudo-random distribution of bars spaced to form a continuous sequence of unique binary words, each indicating a distinct value of the absolute position. The arrangement for reading the absolute track comprises a photoemitter light source placed above the track and a linear type charge transfer detectors CCD located beneath the track, such that the pseudo-random absolute track is read through an image enlarging optical lens positioned in between. The incremental track is read through a sensing unit which comprises a photoemitter and a group of four photodetectors arranged 90 degrees apart in order to produce signals in quadrature for interpolating steps in the track. One drawback of the invention is that it employs an older quadrature detection technique that is can be relatively susceptible to errors and limited in resolution. Another disadvantage of this arrangement is that it uses a large number of components that must be accurately aligned and assembled, and the use of separate sensors to read the absolute and incremental tracks respectively further complicates alignment issues. Moreover, it requires the use of optical lenses, which adds to the cost and complexity of the manufacturing process and precludes its use in some relatively small applications.
Another optical encoder device capable of reading both absolute and incremental tracks is described in scientific article entitled: “High-Resolution Optical Position Encoder with Large Mounting Tolerances”, by K. Engelhardt and P. Seitz, Applied Optics, 1 May 1997. The described encoder detects the absolute track, containing a pseudo-random bit code, and an incremental track made up of equally spaced lines with a specialized ASIC detector chip. A single light source illuminates both tracks to be imaged onto the detector using a positive plastic lens. The specialized detector is made up of multiple sections of which one section comprises a CCD line sensor that has photosensitive elements for reading a single line to determine the absolute code pattern. The other two sections, used to capture the image of the incremental track, each contain two elements with areas shaped as sine square functions. These sine functions have the same frequency as the image of the incremental pattern. The two elements in each section are shifted 180° and the two sections are shifted 90° relative to each other.
The difference between the photocurrents from the two elements in each section will represent a Fourier coefficient for an expansion of the image of the incremental pattern. The phase shift of 90° between the two sections gives that current from the first section will represent a sine coefficient, Isin, and the current from the second section will represent a cosine coefficient, Icos. The phase of the Image of the incremental track is then calculated as:
      φ    =          arctan      ⁡              (                              I            cos                                I            sin                          )              ,
The detector is able to optically calculate a Fourier expansion of the light distribution incremental detection, which enables relatively good resolution. However, a drawback of this configuration is that it is not very flexible i.e. the period of the image of the incremental track must match the spatial frequency of the sinusoidal shape almost exactly or accuracy is lost. This can happen when the disk is jarred causing it to move inadvertently, or by inaccurate disk mounting. This requires the disk be mounted with very little eccentricity and that the sensor must be mounted precisely relative to the disk and the light source. A further disadvantage is that the reference period is fixed by virtue of being embedded in the chip, which cannot change during operation. Moreover, the use of the specialized ASIC detector adds to the overall cost to the encoder and limits production in large quantities.
In view of the foregoing, it is desirable to provide an encoder apparatus and method with high resolution that is relatively less expensive and easier to assemble with fewer components, less sensitive to disk mounting errors, and suitable for applications of small dimensions.